Utilizing mixtures of yttria,magnesia,and lanthanum oxide in manufacture of transparent alumina

ABSTRACT

A METHOD OF MANUFACTURING A POLYCRYSTALLINE TRANSLUCENT ALUMINA CONSISTING OF A HIGH PURITY ALUMINA AND HAVING EXCELLENT OPTICAL TRANSMISSION PROPERTY FOR THE LIGHT IN THE VISIBLE SPECTRUM, THERMAL SHOCK RESISTANCE AND MECHANICAL STRENGTH WHICH COMPRISES THE FOLLOWING STEPS CALCINING ALUMINA POWDERS HAVING A PURITY OF MORE THAN 99.00% BY WEIGHT AS A STARTING MATERIAL AT A TEMPERATURE OF 1,050-1,0250*C. IN AIR, ADDING TO SAID ALUMINA POWDERS 0.05-0.5% BY WEIGHT OF Y2O3, 0.05-0.5% BY WEIGHT OF LA2O3 AND 0.01-0.1% BY WEIGHT OF MGO, MIXING AND THEN MOLDING SAID ALUMINA POWDERS ADDED WITH SAID ADDI-   TIVES, SINTERING AT FIRST SAID MOLDED MIXTURE AT A TEMPERATURE OF 1,200-1,450*C. AND SECONDLY SINTERING IN VACUUM OR A REDUCING ATMOSPHERE SUCH AS HYDROGEN OR DISSOCIATED AMMONIA GAS AT A TEMPERATURE OF 1,60001,800*C.

12, 1974 KAZUO KOBAYASHI ETAL 3,792,142

UTILIZING MIXTURES 0F YTTRIA, NHXGNESIA1 AND LANTHANUM OXIDE INMANUFACTURE OF TRANSPARENT ALUMINA Filed Aug. 18, 1970 5 Sheets-Sheet 5Fig. 3

O l I i I Amount of MgO by weight) Feb. 12, 1974 KAZUO KOBAYASHI ETAL3,792,142

UTILIZING MIXTURES OF YTTRIA, MAGNESIA, AND LANTHANUM OXIDE INMANUFACTURE OF TRANSPARENT ALUMINA Filed Aug. 18, 1970 3 Sheets-5heet EM90 01 wt ---A $203 0.5 wt lo Present Invention 0 05 wt% IGOOOC -B (MgO0.5 wt %)Prior Art Relative Number of Grains o '83 United States Patent3,792,142 UTILIZING MIXTURES 0F YTTRIA, MAGNESIA, AND LANTHANUM OXIDE INMANUFACTURE OF TRANSPARENT ALUMINA Kazuo Kohayashi, Nagoya, and MasayukiKaneno, Tokoname, Japan, assignors to NGK Insulators, Ltd., Mizul1o-ku,Nagoya, Japan Filed Aug. 18, 1970, Ser. No. 64,694 Claims priority,application Japan, Aug. 27, 1969, 44/ 67,338 Int. Cl. C04]: 35/10, 35/64US. Cl. 264--65 6 Claims ABSTRACT OF THE DISCLOSURE A method ofmanufacturing a polycrystalline translucent alumina consisting of a highpurity alumina and having excellent optical transmission property forthe light in the visible spectrum, thermal shock resistance andmechanical strength which comprises the following steps, calciningalumina powders having a purity of more than 99.0% by weight as astarting material at a temperature of 1,0501,250 C. in air, adding tosaid alumina powders 0.05-0.5% by weight of Y O ODS-0.5% by weight of LaO and 0.0l0.1% by weight of MgO, mixing and then molding said aluminapowders added with said additives, sintering at first said moldedmixture at a temperature of 1,200-1,450 C. and secondly sintering invacuum or a reducing atmosphere such as hydrogen or dissociated ammoniagas at a temperature of 1,600- 1,800 C.

This invention relates to methods of manufacturing a translucentsintered body containing aluminium oxide (hereinafter abbreviated toalumina) as its main ingre' dient and having excellent opticaltransmission properties for light in the visible spectrum, mechanicalstrength and thermal shock resistance.

Translucent alumina of a sintered body according to the prior art(described in the US. Pat. Nos. 3,026,177 and 3,026,210) containsalumina as its main ingredient with the addition of magnesia only and isfinally sintered at a high temperature of more than 1,700 O, preferably1,8001,950 C. The high temperature sintering step involves a highmanufacturing cost and hence the translucent alumina is excessively highin price, thus limiting the purposes for which the translucent aluminais to be used. Moreover, owing to the high temperature sintering step,alumina crystal grains grow large in size and prevent pores closed inthe alumina grains from escaping, with the result that not only theoptical transmission property but also the thermal shock resistance,mechanical strength and gas tightness of the translucent alumina areconsiderably decreased and that particularly such reduced mechanicalstrength causes an inevitable breakage of an enveloping tube for a highpressure gas discharge lamp made of the translucent alumina owing to thesudden heat to which it is subjected.

The invention is based on the recognition that the many properties suchas the optical transmission property, thermal shock resistance and themechanical strength of translucent alumina can be considerably improvedand the above disadvantages are removed by using special additives andspecial sintering conditions and particularly provides a. method ofmanufacturing a translucent alumina having the advantages abovementioned, wherein the sintering step is carried out at a temperaturelower than that of the prior art by IOU-200 C.

A feature of the invention is to provide a method of manufacturing apolycrystalline translucent alumina body consisting of a high purityalumina and having excellent thermal shock resistance and mechanicalstrength which comprises the following steps, calcining alumina powdershaving a purity of more than 99.0% by weight at a temperature of1,050-1,250 C. in air, adding to said alumina powders ODS-0.5% by weightof Y O ODS-0.5% by weight of La O and (ml-0.1% by weight of MgO, mixingand then molding and compacting said mixed powders, first sintering saidcompacted mixture at a temperature of 1,200-1,450 C. and secondlysintering it in vacuum or a reducing atmosphere at a temperature of1,600 1,800" C.

The method of manufacturing a translucent alumina body according to thepresent invention will now be described with reference to the sequenceof steps. Gamma alumina powders having a particle size of 0.01-0.1micron and a purity of more than 99.0% by weight are calcined in air ata temperature of 1,050-1,250 C. for 1-10 hours in an electric furnace toconvert at least by weight of the gamma alumina into alpha alumina. Tothis alpha alumina thus calcined are added ODS-0.5% by weight of Y O asa first essential additive, 0.05-0.5% by weight of La o as a secondessential additive and 0.01-0.1% by weight of MgO as a third essentialadditive, percent being taken on the basis of the alumina powders. Thesepowders are thoroughly mixed together in a trommel, for example, withwater in such an amount that the water content in the mixture becomesabout 60%. Then, the mixture is then dehydrated, for example, by avacuum filter, and sufficiently dried at about C. The dried mixture isthen crushed and the crushed material is screened by passing it througha screen of 42-120 meshes to size it. To the material thus sized isadded 1-2% by weight of polyvinyl alcohol as a binder and then is formedinto a body by a cold press. The formed body thus obtained is subjectedto a first sintering at a temperature of 1,200-1,450 C. and then to asecond sintering in vacuum or a reducing atmosphere at a temperature ofl,600-l,800 C. For this second sintering step, it is necessary to use anonoxidizing atmosphere. Hydrogen gas or ammonium decomposed gas for usein the second sintering is subjected beforehand to a drying step inwhich said gas is brought into contact with, for example, an activatedalumina.

In accordance with the present invention, the purity of the aluminapowder used as a starting material must be more than 99.0% by weight.The use of the alumina powder having a purity of less than 99.0% byweight may have the possibility of evaporation of impurities during thehigh temperature sintering step and this evaporation causes not onlypores enclosed in the alumina grains, but also a reaction with aluminato form a second phase thereby giving adverse efi'ects upon thetranslucency and mechanical strength.

Furthermore, alumina powder is desirably in the form of thermallydecomposed alumina from aluminium salts such as aluminium sulphate fromthe point of powder sinterability.

The reason why the alumina powders as a starting material are calcinedat a temperature of 1,050-l,250 C. before sintering is as follows.

It the gamma alumina powders are calcined at a temperature less than1,050 C. in air, the gamma alumina powders do not change the activityand crystallinity, so that during sintering of a mold body the shrinkageof the sintered body is very large and the accuracy of the.

products is remarkably unstable. As the rapid sintering reaction occurs,the difference between the reaction rate of the active alumina particlesand that of inactive particles makes a difference in the rate of graingrowth, thereby causing irregularity in the grain size of the sinteredbody. This causes not only a reduction of the optical transmissionproperty but also a grain boundary crack, thus decreasing the thermalshock resistance and mechanical strength. Moreover, it is not preferableto calcine the gamma alumina at a temperature higher than 1,250 C., forthe particles of the alpha alumina thus obtained are excessively grownand stable, which prevent the sintering reaction. Moreover, theformability and workability become remarkably deteriorated. Thus, it ispreferable to calcine the gamma alumina powders at a temperature of1,050-1,250 C. for 1-10 hours in order to transform them into the alphaalumina powders. Unless the calcining condition is within these ranges,the above mentioned disadvantages occur.

The reason why the additives are added to the gamma alumina powdersafter calcining is as follows. If additives are added to the gammaalumina powders before the calcining, the transformation of the gammaalumina to the alpha alumina occurs locally owing to the local existenceof the additives, whereby the grain growth of the alpha alumina becomeslarger in part. If use is made of alpha alumina as a starting materialand the alpha alumina satisfies the conditions required by the presentinvention, it is a matter of course that the above mentioned step ofcalcining the gamma alumina may be omitted.

The reason why 0.05-0.5% by weight of Y O 0.05- 0.5% by weight of La Oand 0.01-0.1% by weight of MgO are added to the alpha alumina powders isnow explained.

The alumina to which the three additives of 0.3% by weight of Y O 0.1%by weight of 1.3203 and 0.05% by weight of MgO are added shows improvedoptical transmission properties and mechanical strength at the sinteringtemperature of 1,6001,800 C. which are superior to those of the aluminato which 0.5% by weight of MgO are added according to the prior art asshown in FIG. 1 and also the alumina grains are fine and uniform. On thecontrary, the alumina according to the prior art shows an improvedoptical transmission property only at a sintering temperature higherthan 1,800 C., but a reduced mechanical strength and thermal shockresistance owing to an exaggerated grain growth of the alumina grains.

The reason why the polycrystalline translucent alumina according to theinvention can be applied to a discharge lamp to protect it againstbreakage owing to sudden heat change when the lamp is turned on ascompared with the prior art is now considered. If use is made of onlyMgO as an additive, the sintering temperature of more than 1,700 C.increases the vapor pressure of MgO and hence accelerates theevaporation of MgO from the surface of the sintered body, with theresult that the amount of MgO present at the surface of the sinteredbody becomes gradually decreased towards the interior part in dependencewith the lapse of time and hence the eifect of MgO for controlling thegrain growth of alumina crystals becomes decreased so that the rate ofgrain growth of alumina crysetals becomes abnormally large whereby thegrain size of alumina crystal often becomes several ten microns. Thisexaggerated grain growth of the alumina crystal affords the disadvantagethat the optical transmission property and thermal shock resistance areconsiderably reduced.

The theoretical investigation for the improved translucency andmechanical strength by addition of the three additives having diiferenteffects is not clear until now. However the following reasons arededuced.

Y O added as the first essential additive of the three additives servesto accelerate the grain growth of the alumina crystals during sintering.Further, La O added as the second additive serves to prevent the graingrowth of the alumina crystals. MgO added as the third additive servesat a temperature lower than 1,700 C. to spheroidize the aluminaparticles. It is necessary to use all of the three additives. Anexcellent translucent alumina could not be obtained in the absence ofany one of the three additives. It is a well-known fact that MgO reactswith A1 0 to produce spinel near the grain boundary which prevents theexaggerated grain growth in the sintering step. But MgO is evaporatedwith increase of the sintering temperature and time to decrease thethickness of the spinel layer and hence to cause grain growth. Thepresence of the additives such as La O together with MgO contributes tostabilize the spinel and hence the spinel controls the grain growth upto a high temperature so that the exaggerated grain growth can not occurwhereby alumina crystals remain fine and uniform.

Y O serves to accelerate the grain growth of the alumina crystals at thebeginning of the sintering step and to make the molded body of aluminabecome dense rapidly. But, since the sintering temperature where Y Oacts is low, the exaggerated grain growth of the alumina does not occur.Moreover, during the sintering step effected at a temperature of1,6001,800 C., MgO is gradually evaporated and its efiect ofspheroidizing the alumina particles becomes decreased. But, up to thistime, MgO already has served to spheroidize the alumina particles andfurther increase the uinformity of the alumina grain.

Furthermore, La O and Y 0 influence the refractive index of the sinteredalumina. It is well-known that the translucency of the sintered aluminawith only MgO as an additive is decreased by the amount ofmagnesiaalumina spinel at the grain boundary. This fact comes from thedifference of the refractive index between A1 0 and magnesia-aluminaspinel and the addition of Y O and L21 O make the refractive index ofgrain boundary; products close to that of A1 0 That is why thetranslucent alumina of the present invention has a high translucencycompared to the prior art.

The reason why the amounts of the additives are limited is as follows.

(1) The reason why the amount of Y O is limited to 0.05-0.5% by weightis that addition of less than 0.05% by weight of Y O makes it difficultto add uniformly in the alumina powders, and that addition of more than0.5 by weight results in an excessive acceleration of the grain growthat the beginning of the sintering reaction, thereby making the grainsize of the alumina crystals in the final article irregular. The reasonwhy the amount of La O is limited to 0.050.5% by weight is that additionof less than 0.05 by weight is ineifective for the same reason as thatmentioned above and that addition of more than 0.5 by weight causes anadverse influence upon the thermal shock resistance and anti-corrosionproperty for alkali metal of the translucent alumina.

(2) The reason why the amount of MgO is limited to 0.01-O.1% by weightis that addition of less than 0.01% by weight of magnesia renders itdifficult to add MgO uniformly, which causes a risk of local absence ofmagnesia in the alumina powders and that addition of more than 0.1 byweight of MgO causes the above mentioned evaporation during thesintering steps which only pollutes the furnace and is ineffective forthe properties of translucent alumina. The composition ranges of theadditives according to the invention makes it possible to manufacture atranslucent alumina having desired properties above mentioned. Theadditives may be added to the alumina powders not only in the form ofoxides, but also in the form of salts such as sulphate, nitrates,chloride which can be changed into oxides by the sintering steps.

FIG. 4 indicates the good composition in phase diagram for the opticaltransmission when the pellets with A3 thickness were fired at 1,350 C.for 2 hours and 1,750 C. for 2.5 hours in dry hydrogen. In this diagram,the rate of Y O to La O is within 05-15 Dark area shows above 60% oftransmission and cross hatched area shows above 50%.

The reason why the first sintering step is effected at a temperature of1,200-1,450 C. and the second sintering step is effected in vacuum or areducing atmosphere at a temperature of 1,600-1,800 C. is now described.

In general, the sintering reaction is effected as follows. The graingrowth of alumina crystals begins at that portion of the aluminaparticles with large surface energy and then follows another portion ofthe alumina particles with small surface energy to make the aluminacrystals dense with sintering. Subsequently the rate of grain growth ofthe alumina crystals having a large surface energy is selectivelyaccelerated with increasing the sintering temperature thus preventingthe pores from escaping, the socalled exaggerated grain growth ofalumina. Thus, it is quite important to make the particle size uniformat the beginning of the sintering reaction and prevent a grain growth atthe latter half period.

In accordance with the invention, the first sintering is effected at atemperature of 1,200-1,450 C. for 1-5 hours to make the particle sizegrown at the beginning uniform by the effect of the additives,particularly of Y O and further the sintering temperature is maintainedat a temperature of l,600-1,800 C. with preventing the grain growth bythe effect of La O to make it possible to obtain a sintered articleconsisting of uniform and fine grains.

The sintering time must be varied in depedence upon the kind and form ofarticles to be obtained and also upon the sintering temperature andhence cannot be determined a single time. For an article having athickness of the order of 1 mm., it is most preferable to sinter it at atemperature of 1,350 C. for 5 hours and then at a temperature of 1,700C. for about 5 hours.

The following examples are given in illustration of this invention andare not intended as limitations thereof.

For a better understanding of the invention, reference is taken to theaccompanying drawings, wherein:

FIGS. 1A, 1B and 1C show the relations between the sintering temperatureon the one hand and the translucency, mechanical strength and averagegrain size of alumina crystal on the other hand, the curves showing theresults obtained by the invention being compared with those obtained bythe prior art:

FIG. 2 shows the characteristic curves illustrating the relationsbetween the sintering temperature, the additives and the grain sizedistribution of the alumina article obtained by the method according tothe invention and that obtained by the prior art;

FIG. 3 is a comparison graph of the flexural strength between thetranslucent alumina obtained by the method according to the inventionand that obtained by the prior art; and

FIG. 4 shows the phase diagram, representing excellent opticaltransmission properties for light in the visible spectrum.

EXAMPLE 1 Gamma alumina powders having a high purity of more than 99.0%by weight were calcined in air at a tempera ture of 1,150 C. for hoursin an electric furnace to convert them into alpha alumina powders. Toone portion of the alpha alumina powders thus calcined 0.5% by weight ofmagnesia was added in accordance with the prior art, and to anotherportion 0.05% by weight of magnesia, 0.3% by weight of Y O and 0.1% byweight of La O were added in accordance with the presence invention.

Each of these two kinds of powders was sufficiently mixed respectively,and then cold pressed by applying 1.4 ton/cm. to obtain pellets with adimension of 30 mm. dia x 1.5 mm. thickness and hexagonal bars with alength of 50 mm. The pellets and hexagonal bars thus obtained weresintered in dry hydrogen gas at a temperature of 1,250 C. for 5 hoursand then sintered at different temperatures of 1,500, 1,600, 1,700,1,800 C., respectively, for 5 hours.

FIG. 1A shows the optical transmission property of the translucentalumina pellets measured by the photometric integrating sphere, asmentioned, for example, in Rosa, E. B. and Taylor, A. H.: TheoryConstruction and Use of the Photometric Integrating Sphere, Sci. PaperNo. 447, Bull. Bur. Stand, Sept. 26, 1921.

FIG. 1B shows also the flexural strength of the translucent alumina ofthe hexagonal bars measured by the three points supporting method on thebasis of ASTM C l33-37T and the average grain size of alumina crystalsmeasured by a microscopic observation of the cross-section. As seen fromthe results shown in FIG. 1A, translucent alumina to which 0.5% magnesiawas added showed an improved optical transmission property at thesintering temperature of 1,800" C., but the flexural strength wasgreatly reduced at the same temperature. On the contrary, thetranslucent alumina with the three additives in accordance with thepresent invention showed an improved optical transmission property at asintering temperature in the range of 1,6001,800 C. and its flexuralstrength in the same sintering temperature range was not so muchreduced. The high flexural strength provides good thermal shockresistance.

EXAMPLE 2 To alpha alumina powders obtained by calcining gamma aluminahaving a purity more than 99.0% were added magnesium nitrate, yttriumnitrate and lanthanum nitrate solutions reactable to produce 0.5% byweight of magnesia on the one hand and also reactable to produce 0.1% byweight of magnesia, 0.5% by weight of Y 0 and 0.5% by weight of La O onthe other hand to obtain two kinds of powders. Each of these two powderswas sufficiently mixed respectively in a trommel and then dried invacuum. The dried material was then press molded into a pellet by ametal mold with 30 mm. diameter. The pellets thus obtained weresimultaneously sintered at a temperature of 1,400" C. for 3 hoursbeforehand and then sintered at different temperature of 1,600, 1,700and 1,800 C., respectively, for 5 hours with flowing hydrogen gas byactivated alumina. The translucent alumina pellets were obtained with adimension of 30 mm. dia. x 1.0 mm. thickness. The optical transmissionproperty of the translucent alumina pellets by the present invention wasmore than while that of translucent alumina pellet by the prior art was80% when sintered at 1,800" C. but was less than 80% when sintered atthe other range of temperature. The grain size distribution of thepellets sintered at 1,600 C. and 1,800 C. was measured by the microscopeafter sufficiently polished. The results are shown in FIG. 2. In FIG. 2,the characteristic curves for the pellets obtained by the prior art areshown by dotted lines, while those of the present invention are shown byfull lines.

'As seen from the test results shown in FIG. 2, the translucent aluminawith 0.5% magnesia showed an exaggerated grain growth and was irregularin uniformity when sintered at l,800 'C. On the contrary, thetranslucent alumina by the present invention. showed not so exaggeratedgrain growth of alumina when sintered at 1,800 C. and an improveduniformity.

EXAMPLE 3 To alpha alumina powders calcined in the same manner asExample 1 were added additives as shown in the following Table 1. Eachof these powders was thoroughly mixed together nad the mixture thusobtained was press molded into a pellet by a metal mold having adiameter of 30 mm. The pellets thus molded were simultaneously sinteredin air at 1,350 C. for 5 hours, and then sintered again in vacuum of 3 x10 mm. Hg at 1,700" C. for 10 hours to obtain translucent pellets eachhaving a dimension of about 23 mm. dia x 1 mm. thickness. The opticaltransmission property of the translucent pellet is shown in thefollowing Table 1. The samples with or without added 0.5% by weight ofmagnesia are shown in the Table 1 as compared reference.

8 tives, but can be obtained by addition of the three additives.

The translucent ceramics such as alumina may be used TABLE 1 for anenveloping tube of a high pressure discharge lamp, gl g gfl etc., whichrequires excellent optical transmission prop- Sample property erty andthermal and mechanical strength. The sintered number Additive (percent)(pemnt) bodies by the present invention are superior in these 36properties to the prior art and can easily be manufactured 2% in auniform and less expensive manner. Particularly, the 81 presentinvention provides the important advantage that 53 the sintered body ofthe present invention can be applied 82 to discharge lamps so as toprotect them against breakage owing to sudden heat change to which theyare subjected, EXAMPLE 4 such breakage being the fatal disadvantage ofthe prior To calcined alpha alumina powders were addg (1 w c1aimedis0.2% b wei ht of ma nesia onl and (2) 0.0 0 y Weight maggnesia, 0.25% bywaigilllt of Yzoa and 0.25% 1. A method of manufacturing a translucentalumina by weight of La O Each of these two kinds of powders body havmgan averflge cry Sta1 slze of 1040 mlcPns was molded into a pipe-shapedmold. The pipe-shaped ma- F optlcal transmlsslon P P TY for llghtterials thus obtained were sintered at 1,300 C. for 5 hours 20 m theYlslble Spectrum}, themlal Shock Teslstance and and then sintered againin dry hydrogen at 1,800 C. to mechanical Strength Winch comprlses lfollowing p obtain a translucent alumina pipe having a dimension ofcalclnlng gamma fllllmlfla Powders having a Partlcle Size 10 mm. out.dia x 8 mm. in. dia x 50 mm. length. The of 9 mlcron and a P y of morethan 990% y optical transmission property of the translucent aluminaWelght F a temperatufepf 5 C. so as to conpipe and the results ofthermal shock tests are shown in Vel't Sald gamma alllfnlna Powders Intoalpha alumina the following Table 2. The thermal shock test was carriedP addlng t0 8 pha alumina poWders and mixout as follows. The sample wasrapidly put into an electric i g th rewith 0.050.5% by weight of Y OODS-0.5% furnace held at 1,200 C. and kept therein for 2 minutes. byweight of La O and 0.0l0.1% by weight of MgO, or Then, the sample wasremoved out of the electric furnace salts thermally decomposable to suchoxides, compacting and rapidly cooled. Such sequence of steps wasrepeated. the mixture, first sintering said compacted mixture at a TABLE2 Optical Times of thermal shock test; transmission Sample property No.Additive (percent) 1 2 3 4 5 6 7 8 9 10 (percent) 1 MgO (0.2) 0 x 80 2MgO (0.2) o o x s3 3 MgO (0.05), YzO; (0.25), LaaO; (0.25) o o o 0 o o oo o o 92 4 MgO (0.05), Yzos (0.25), 119.203 (0.25)..-- o o 0 o o o o o oo 96 In Table 2 0 shows that no crack occurs in the sample and x showsthat cracks occur in the sample.

As seen from Table 2, the sample obtained by the present invention issuperior in its thermal shock resistance which is a key of a dischargelamp. The grain size distribution of the sample obtained by the priorart was 30-40 microns, while that of the sample obtained by the presentinvention was 10-15 microns.

EXAMPLE 5 To calcined alpha alumina powders were added (1)0- 0.15% byweight of magnesia, (2) 0.2% by weight 1.3.203, 00.1% by weight ofmagnesia, (3) 0.2% by weight of Y O 00.1% by weight of magnesia, and (4)0.2% by weight of La O 0.2% by weight of Y O 0-0.l% by weight ofmagnesia. Each of these four kinds H:V04 was mixed respectively and thencold pressed by applying a pressure of 1.4 ton/cm. into a hexagonal barhaving a length of mm. and a pellet having a dimension of 30 mm. dia x1.5 mm. thickness. Each of these hexagonal bars and pellets was sinteredin dehydrated hydrogen gas at 1,300 C. for 5 hours and then againsintered at 1,700 C. for 5 hours. FIG. 3 shows the flexural strength ofthe translucent alumina hexagonal bar measured by three three pointsupporting method. As seen from FIG. 3 the samples to which La O+Y O+MgO was added showed far superior in flexural strength to the sample towhich MgO only was added and to the samples to which Y O +MgO were addedand La O +MgO. This is explained by the fact that the samples accordingto the present invention consist of smaller size grains that those ofsamples according to the prior art. As explained hereinbefore, asutfucient mechanical strength cannot be obtained by addition of MgOonly or two of the three additemperature of 1,2001,450 C. for 15 hrs.and secondly sintering in a vacuum or in an atmosphere of hydrogen ordissociated ammonia gas at a temperature of 1,600 1,800" C. for 1-10hrs.

2. A method as claimed in claim 1, wherein said Y O La O and MgO areadded in the form of salts or aqueous soluition thereof which whensintered are converted into on es.

3. A method as claimed in claim 1, wherein said gamma alumina powdersare calcined in air in an electric furnace at a temperature of1,050-1,250 C. for 11() hrs. to convert at least by weight of the gammaalumina into alpha alumina.

4. A method as claimed in claim 1, wherein water is added to saidalumina powders mixed with said additives and the mixture is thereafterdehydrated, crushed, screened and mixed with a binder prior to saidcompacting.

5. A method for manufacturing a translucent alumina body having anaverage crystal size of l0-20 microns (-20 excellent opticaltransmission property for light in the visible spectrum, thermal shockresistance and mechanical strength which comprises the following steps,preparing a mixture consisting of ODS-0.5% by weight of Y O 0.5-0.5 byweight of La O '0.010.1% by weight of MgO, or salts thermallydecomposable to such oxides, and the remainder of alpha alumina powdershaving a particle size of 001-01 micron and a purity of more than 99.0%by weight, compacting the mixture, first sintering said compactedmixture at a temperature of 1,2001,450 C. for 1-5 hrs. and secondlysintering in a vacuum or in an atmosphere of, hydrogen or dissociatedammonia gas at a temperature of 1,600-1,800 C. for 1-10 hrs.

6. A method as claimed in claim 5, wherein said Y O La O and Mgo areadded in the form of salts or aqueous 9 solution thereof which whensintered are converted into oxides.

References Cited UNITED STATES PATENTS 3,711,585 1/1973 Muta et a1.264-65 3,026,210 3/1962 Coble 10639 3,377,176 4/1968 Walkodofi et a1.264--65 10 OTHER REFERENCES W. J. Smothers et a1., Sintering and GrainGrowth of Alumina, Jour. Amer. Cer. -Coc., December 1954, at 588595.

JOHN H. MILLER, Primary Examiner US. Cl. X.R. 10665; 264--66 UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,792,142Dated February 12, 1974 Inventor) Kazuo KOBAYASHI and Masayuki KANENO Itis certified that error appears in the aboveidentified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column1 3 ,,line 58 delete "crysetals" and insert I 1 -crystals-n vColumn 5, line 27 after "determined" insert ---at---;

' line 64 delete "presence" andinsert -present--; Column 6, lin'e 64after "as" first occurrence, insert ----in-; line 66 delete "nad" andinsert f -and-w Columnpj, line 57 delete H:V04" and insert -o f Q 7"powdersline 66, delete "three" first occurrence and insert --the; Iline 67, delete "La 0" and insert -La 0 ,line 67, delete "was" andinsert --were--: line 70, delete "were added"; line 70, after "MgO"second occurrence, insert t :.---were added;

- USCOMM-DC scan-Pub U.S. GOVIINIIINT I'IINIIIIG OIIICF "I! 0-3064 FORMPO-105 Q(10-69) l STATES PATENT OFFIQE V CER A OF CORRECTION Patent 'No.7 3 142 Dated. F F Y w 1: Q

PAGE 2 Ir went ofl s) Ka'zuo KOBAYASHI and Masayuki KANE-N0 A It iscezfti'fied that error appears in lthe above-identified patent and thatsaid Lgtte'rs Patent are hereby "corrected as shown below:

Column 8 ,jf'f1ine l7 delete 'miccrms' and in'siejt "micronsf fl in 65 ale'gg "0.15-0.52" and j j'j-, o. 05-0; 5%;

Signed i t his zoth la (SEAL) Attefi? I I Attgsting E93 9 "comi ssidriex i .blfifPa'tentsl

